GSM – Architecture, Protocols and ServicesGSM – Architecture, Protocols and Services Third Edition Jörg Eberspächer Technische Universität München, Germany Hans-Jörg Vögel

GSM – Architecture,Protocols and Services

Third Edition

Jörg Eberspächer

Technische Universität München, Germany

Hans-Jörg Vögel

BMW Group Research & Technology, Germany

Christian Bettstetter

University of Klagenfurt, Austria

Christian Hartmann


A John Wiley and Sons, Ltd, Publication

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GSM – Architecture, Protocols and ServicesThird Edition

GSM – Architecture,Protocols and Services

Third Edition

Jörg Eberspächer


Hans-Jörg Vögel

BMW Group Research & Technology, Germany

Christian Bettstetter

University of Klagenfurt, Austria

Christian Hartmann


A John Wiley and Sons, Ltd, Publication

This English language edition first published 2009c© 2009 John Wiley & Sons Ltd

Originally published in the German language by B.G. Teubner GmbH as “Jörg Eberspächer/Hans-JörgVögel/Christian Bettstetter: GSM Global System for Mobile Communication. 3. Auflage(3rd edition).”c© B.G. Teubner GmbH, Stuttgart/Leipzig/Wisbaden 2001

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Library of Congress Cataloging-in-Publication Data

Eberspaecher, Joerg.GSM, Global System for Mobile Communication. EnglishGSM : architecture, protocols and services / Joerg Eberspaecher . . . [et al.]. – 3rd ed.

p. cm.Prev. ed.: GSM switching, services, and protocols, 2001.ISBN 978-0-470-03070-7 (cloth)

1. Global system for mobile communications. I. Eberspaecher, J. (Joerg) II. Title.TK5103.483.E2413 2008621.3845’6–dc22

2008034404

A catalogue record for this book is available from the British Library.

ISBN 978-0-470-03070-7 (H/B)

Set in 10/12pt Times by Sunrise Setting Ltd, Torquay, UK.Printed in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire.

www.wiley.com

Contents

Preface xi

1 Introduction 11.1 The idea of unbounded communication . . . . . . . . . . . . . . . . . . . . 11.2 The success of GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Classification of mobile communication systems . . . . . . . . . . . . . . . . 31.4 Some history and statistics of GSM . . . . . . . . . . . . . . . . . . . . . . . 51.5 Overview of the book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 The mobile radio channel and the cellular principle 92.1 Characteristics of the mobile radio channel . . . . . . . . . . . . . . . . . . . 92.2 Separation of directions and duplex transmission . . . . . . . . . . . . . . . 12

2.2.1 Frequency Division Duplex . . . . . . . . . . . . . . . . . . . . . . 132.2.2 Time Division Duplex . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Multiple access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.1 Frequency Division Multiple Access . . . . . . . . . . . . . . . . . . 142.3.2 Time Division Multiple Access . . . . . . . . . . . . . . . . . . . . 152.3.3 Code Division Multiple Access . . . . . . . . . . . . . . . . . . . . 172.3.4 Space Division Multiple Access . . . . . . . . . . . . . . . . . . . . 18

2.4 Cellular principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.4.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.4.2 Carrier-to-interference ratio . . . . . . . . . . . . . . . . . . . . . . 242.4.3 Formation of clusters . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4.4 Traffic capacity and traffic engineering . . . . . . . . . . . . . . . . 262.4.5 Sectorization of cells . . . . . . . . . . . . . . . . . . . . . . . . . . 282.4.6 Spatial filtering for interference reduction (SFIR) . . . . . . . . . . . 31

3 System architecture and addressing 433.1 System architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.2 The SIM concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3 Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.3.1 International mobile station equipment identity . . . . . . . . . . . . 463.3.2 International mobile subscriber identity . . . . . . . . . . . . . . . . 473.3.3 Mobile subscriber ISDN number . . . . . . . . . . . . . . . . . . . . 473.3.4 Mobile station roaming number . . . . . . . . . . . . . . . . . . . . 48

vi CONTENTS

3.3.5 Location area identity . . . . . . . . . . . . . . . . . . . . . . . . . 493.3.6 Temporary mobile subscriber identity . . . . . . . . . . . . . . . . . 493.3.7 Other identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.4 Registers and subscriber data . . . . . . . . . . . . . . . . . . . . . . . . . . 503.4.1 Location registers (HLR and VLR) . . . . . . . . . . . . . . . . . . 503.4.2 Security-related registers (AUC and EIR) . . . . . . . . . . . . . . . 513.4.3 Subscriber data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.5 Network interfaces and configurations . . . . . . . . . . . . . . . . . . . . . 533.5.1 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.5.2 Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4 Air interface – physical layer 574.1 Logical channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4.1.1 Traffic channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.1.2 Signaling channels . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.1.3 Example: connection setup for incoming call . . . . . . . . . . . . . 614.1.4 Bit rates, block lengths and block distances . . . . . . . . . . . . . . 614.1.5 Combinations of logical channels . . . . . . . . . . . . . . . . . . . 62

4.2 Physical channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.2.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.2.2 Multiple access, duplexing and bursts . . . . . . . . . . . . . . . . . 654.2.3 Optional frequency hopping . . . . . . . . . . . . . . . . . . . . . . 694.2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.3 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.3.1 Frequency and clock synchronization . . . . . . . . . . . . . . . . . 714.3.2 Adaptive frame synchronization . . . . . . . . . . . . . . . . . . . . 73

4.4 Mapping of logical onto physical channels . . . . . . . . . . . . . . . . . . . 754.4.1 26-frame multiframe . . . . . . . . . . . . . . . . . . . . . . . . . . 774.4.2 51-frame multiframe . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.5 Radio subsystem link control . . . . . . . . . . . . . . . . . . . . . . . . . . 804.5.1 Channel measurement . . . . . . . . . . . . . . . . . . . . . . . . . 814.5.2 Transmission power control . . . . . . . . . . . . . . . . . . . . . . 864.5.3 Disconnection due to radio channel failure . . . . . . . . . . . . . . 874.5.4 Cell selection and operation in power conservation mode . . . . . . . 89

4.6 Channel coding, source coding and speech processing . . . . . . . . . . . . . 914.7 Source coding and speech processing . . . . . . . . . . . . . . . . . . . . . . 924.8 Channel coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

4.8.1 External error protection: block coding . . . . . . . . . . . . . . . . 984.8.2 Internal error protection: convolutional coding . . . . . . . . . . . . 1034.8.3 Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074.8.4 Mapping onto the burst plane . . . . . . . . . . . . . . . . . . . . . 1134.8.5 Improved codecs for speech services: half-rate codec, enhanced

full-rate codec and adaptive multi-rate codec . . . . . . . . . . . . . 1154.9 Power-up scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

CONTENTS vii

5 Protocols 1215.1 Protocol architecture planes . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215.2 Protocol architecture of the user plane . . . . . . . . . . . . . . . . . . . . . 123

5.2.1 Speech transmission . . . . . . . . . . . . . . . . . . . . . . . . . . 1235.2.2 Transparent data transmission . . . . . . . . . . . . . . . . . . . . . 1265.2.3 Nontransparent data transmission . . . . . . . . . . . . . . . . . . . 127

5.3 Protocol architecture of the signaling plane . . . . . . . . . . . . . . . . . . 1305.3.1 Overview of the signaling architecture . . . . . . . . . . . . . . . . . 1305.3.2 Transport of user data in the signaling plane . . . . . . . . . . . . . . 139

5.4 Signaling at the air interface (Um) . . . . . . . . . . . . . . . . . . . . . . . 1405.4.1 Layer 1 of the MS-BTS interface . . . . . . . . . . . . . . . . . . . . 1405.4.2 Layer 2 signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425.4.3 Radio resource management . . . . . . . . . . . . . . . . . . . . . . 1465.4.4 Mobility management . . . . . . . . . . . . . . . . . . . . . . . . . 1525.4.5 Connection management . . . . . . . . . . . . . . . . . . . . . . . . 1565.4.6 Structured signaling procedures . . . . . . . . . . . . . . . . . . . . 1605.4.7 Signaling procedures for supplementary services . . . . . . . . . . . 1615.4.8 Realization of SMS . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

5.5 Signaling at the A and Abis interfaces . . . . . . . . . . . . . . . . . . . . . 1665.6 Security-related network functions: authentication and encryption . . . . . . 173

5.6.1 Protection of subscriber identity . . . . . . . . . . . . . . . . . . . . 1735.6.2 Verification of subscriber identity . . . . . . . . . . . . . . . . . . . 1735.6.3 Generating security data . . . . . . . . . . . . . . . . . . . . . . . . 1755.6.4 Encryption of signaling and payload data . . . . . . . . . . . . . . . 176

5.7 Signaling at the user interface . . . . . . . . . . . . . . . . . . . . . . . . . . 179

6 Roaming and handover 1836.1 Mobile application part interfaces . . . . . . . . . . . . . . . . . . . . . . . 1836.2 Location registration and location update . . . . . . . . . . . . . . . . . . . . 1846.3 Connection establishment and termination . . . . . . . . . . . . . . . . . . . 188

6.3.1 Routing calls to MSs . . . . . . . . . . . . . . . . . . . . . . . . . . 1886.3.2 Call establishment and corresponding MAP procedures . . . . . . . . 1916.3.3 Call termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1956.3.4 MAP procedures and routing for short messages . . . . . . . . . . . 195

6.4 Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1976.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1976.4.2 Intra-MSC handover . . . . . . . . . . . . . . . . . . . . . . . . . . 1996.4.3 Decision algorithm for handover timing . . . . . . . . . . . . . . . . 1996.4.4 MAP and inter-MSC handover . . . . . . . . . . . . . . . . . . . . . 205

7 Services 2117.1 Classical GSM services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

7.1.1 Teleservices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2117.2 Popular GSM services: SMS and MMS . . . . . . . . . . . . . . . . . . . . 212

7.2.1 SMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2127.2.2 EMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

viii CONTENTS

7.2.3 MMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2137.3 Overview of GSM services in Phase 2+ . . . . . . . . . . . . . . . . . . . . 2147.4 Bearer and teleservices of GSM Phase 2+ . . . . . . . . . . . . . . . . . . . 215

7.4.1 Advanced speech call items . . . . . . . . . . . . . . . . . . . . . . 2157.4.2 New data services and higher data rates: HSCSD, GPRS and EDGE . 220

7.5 Supplementary services in GSM Phase 2+ . . . . . . . . . . . . . . . . . . . 2217.5.1 Supplementary services for speech . . . . . . . . . . . . . . . . . . . 2217.5.2 Location service . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

7.6 Service platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2227.6.1 CAMEL: GSM and INs . . . . . . . . . . . . . . . . . . . . . . . . 2237.6.2 Service platforms on the terminal side . . . . . . . . . . . . . . . . . 224

7.7 Wireless application protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 2267.7.1 Wireless markup language . . . . . . . . . . . . . . . . . . . . . . . 2267.7.2 Protocol architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 2277.7.3 System architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 2307.7.4 Services and applications . . . . . . . . . . . . . . . . . . . . . . . . 231

8 Improved data services in GSM: GPRS, HSCSD and EDGE 2338.1 GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

8.1.1 System architecture of GPRS . . . . . . . . . . . . . . . . . . . . . . 2348.1.2 Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2378.1.3 Session management, mobility management and routing . . . . . . . 2388.1.4 Protocol architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 2428.1.5 Signaling plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2478.1.6 Interworking with IP networks . . . . . . . . . . . . . . . . . . . . . 2498.1.7 Air interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2508.1.8 Authentication and ciphering . . . . . . . . . . . . . . . . . . . . . . 2578.1.9 Summary of GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

8.2 HSCSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2608.2.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2618.2.2 Air interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2618.2.3 HSCSD resource allocation and capacity issues . . . . . . . . . . . . 263

8.3 EDGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2648.3.1 The EDGE concept . . . . . . . . . . . . . . . . . . . . . . . . . . . 2648.3.2 EDGE physical layer, modulation and coding . . . . . . . . . . . . . 2658.3.3 EDGE: effects on the GSM system architecture . . . . . . . . . . . . 2668.3.4 ECSD and EGPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . 2678.3.5 EDGE Classic and EDGE Compact . . . . . . . . . . . . . . . . . . 268

9 Beyond GSM and UMTS: 4G 269

Appendices 271

A Data communication and networking 273A.1 Reference configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273A.2 Overview of data communication . . . . . . . . . . . . . . . . . . . . . . . . 274

CONTENTS ix

A.3 Service selection at transitions between networks . . . . . . . . . . . . . . . 277A.4 Bit rate adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277A.5 Asynchronous data services . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

A.5.1 Transparent transmission in the mobile network . . . . . . . . . . . . 280A.5.2 Nontransparent data transmission . . . . . . . . . . . . . . . . . . . 284A.5.3 PAD access to public packet-switched data networks . . . . . . . . . 286

A.6 Synchronous data services . . . . . . . . . . . . . . . . . . . . . . . . . . . 288A.6.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288A.6.2 Synchronous X.25 packet data network access . . . . . . . . . . . . 289

A.7 Teleservices: fax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

B Aspects of network operation 295B.1 Objectives of GSM NM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295B.2 Telecommunication management network . . . . . . . . . . . . . . . . . . . 297B.3 TMN realization in GSM networks . . . . . . . . . . . . . . . . . . . . . . . 300

C GSM Addresses 305

D List of Acronyms 307

References 313

Index 317

PrefaceThe GSM family (GSM, GPRS, EDGE) has become one of the most successful technicalinnovations in history. As of June 2008, more than 2.9 billion subscribers were using GSM,corresponding to a market share of more than 81%, and its story continues, even now, despitethe introduction and development of next-generation systems such as IMT-2000 or UMTS(3G) and even systems beyond 3G, dubbed IMT-Advanced.

At the same time, wireless local area networks have substantially expanded the wirelessmarket, sometimes drawing market share from GPRS and 3G (e.g. in public WiFi hotspots),sometimes coexisting (e.g. in UMTS home routers used as a replacement for fixed wireconnections). However, these are used typically for low mobility applications. Mobilecommunication with all of its features and stability has become increasingly important:cellular and GSM technology, plus, of course, lately 3G, GSMs sister technology, so-to-say.

Another impressive trend has emerged since our last edition: the permanent evolutionin the handheld market, producing fancy mobile phones with cameras, large memory, MP3players, Email clients and even satellite navigation. These features enable numerous nonvoiceor multimedia applications, from which, of course, only a subset is or will be successful onthe market.

In this third edition, we concentrate again on the architecture, protocols and operationof the GSM network and outline and explain the innovations introduced in recent years.The main novelties in this book are the presentation of capacity enhancement methods suchas sectorization, the application of adaptive antennas for Spatial Filtering for InterferenceReduction (SFIR) and Space Division Multiple Access (SDMA), a detailed introductionto HSCSD and EDGE for higher data rates, and an update of the available GSM services,specifically introducing the Multimedia Messaging Service (MMS).

We are happy to have received, over the past few years, many constructive comments,and a lot of praise and encouragement. The book has obviously been successfully used byprofessionals (especially people beginning careers in the cellular network business) but alsoby students including our own who use it as a textbook enhancing their course material.

Our author team has been enlarged with the addition of Dr. Christian Hartmann, anassistant professor at Technische Universität München, who took most of the load for thisedition.

We thank all of the involved staff from Wiley who convinced us to prepare this updatedversion of a book that will hopefully be as successful over the next few years as in the past.

Jörg EberspächerHans-Jörg Vögel

Christian BettstetterChristian Hartmann

Munich

1

Introduction

1.1 The idea of unbounded communication

Communication everywhere, with everybody, and at any time – that was the dream and goalof researchers, engineers and users, since the advent of the first wireless communication sys-tems. Today it feels like we have almost reached that goal. Digitalization of communicationsystems, enormous progress in microelectronics, computers and software technology, theinvention of efficient algorithms and procedures for compression, security and processing ofall kinds of signals, as well as the development of flexible communication protocols haveall been important prerequisites for this progress. Today, technologies are available thatenable the realization of high-performance and cost-effective communication systems formany application areas.

Using current wireless communication systems, the most popular of which is GSM(Global System for Mobile Communication), we see that we have the freedom to notonly roam within a network, but also between different networks, and that we can in factcommunicate (almost) everywhere (unless we are in one of the rare spots still without GSMcoverage today), with (almost) everybody (unless our desired communication partner is inone of the rare spots mentioned above or chooses not to be reachable), and at (almost) anytime (unless we forgot to pay our last phone bill and the operator decides to lock us out). Ifthere is one major aspect still missing in order to make our wireless experience flawless, itis the large (albeit diminishing) gap between data rates available through wireless servicesand those available through wired services, such as Digital Subscriber Line (xDSL). This andthe limited capability of data representation at the mobile terminal (mostly due to the limitedsize of mobile phones) is one of the main challenges for future developments in wirelesscommunication.

Let us now briefly take a look at the functionalities, which enable us to move and roam sofreely in GSM systems: terminal mobility and personal mobility.

In the case of terminal mobility, the subscriber is connected to the network in a wirelessway – via radio- or light-waves – and can move with their terminal freely, even during a

GSM – Architecture, Protocols and Services Third Edition J. Eberspächer, H.-J. Vögel, C. Bettstetter and C. Hartmannc© 2009 John Wiley & Sons, Ltd

2 GSM – ARCHITECTURE, PROTOCOLS AND SERVICES

communication connection. The degree of mobility depends on the type of mobile radionetwork. The requirements for a cordless in-house telephone are much less critical than for amobile telephone that can be used in a car or train. If mobility is to be supported across thewhole network (or country) or even beyond the network (or national) boundaries, additionalswitching technology and administrative functions are required, to enable the subscribers tocommunicate in wireless mode outside of their home areas.

Such extended network functions are also needed to realize personal mobility anduniversal reachability. This is understood to comprise the possibility of location-independentuse of all kinds of telecommunication services, including fixed and wireless networks. Theuser identifies themselves (the person), e.g. by using a chip card, at the place where they arecurrently staying and have access to the network. There, the same communication servicescan be used as at home, limited only by the properties of the local network or terminal used.A worldwide unique and uniform addressing system is an important requirement for personalmobility.

In the digital mobile communication system GSM, which is the subject of this book,terminal mobility is the predominant issue. Wireless communication has become possiblewith GSM in any town, any country and even on any continent.

GSM technology contains the essential intelligent functions for the support of personalmobility, especially with regards to user identification and authentication, and for thelocalization and administration of mobile users. Here it is often overlooked that in mobilecommunication networks by far the largest part of the communication occurs over the fixednetwork part, which interconnects the radio stations (base stations). Therefore, it is nosurprise that in the course of further development and evolution of the telecommunicationnetworks, a lot of thought has been given to the convergence of fixed and mobile networks.

In the beginning, GSM was used almost exclusively for speech communication; however,the Short Message Service (SMS) soon became extremely popular with GSM users: severalbillion text messages are being exchanged between mobile users each month. In the meantime, additional data services have been realized, most notably the High Speed CircuitSwitched Data (HSCSD) and the General Packet Radio Service (GPRS), which enableimproved data rate performance by allowing for more than one GSM timeslot to be usedby a terminal for a service at a time. The driving factor for new (and higher bandwidth) dataservices obviously is wireless access to the Internet. To this end, the Wireless ApplicationProtocol (WAP) is also explained in this book. These additions are already working towardsclosing the gap between wireless and fixed networks that we discussed above.

A further step was the introduction of third-generation (3G) mobile communicationnetworks. The 3G networks, known as the Universal Mobile Telecommunication System(UMTS) in Europe and as the International Mobile Telecommunication System 2000 (IMT-2000) worldwide, have already been introduced. However, the implementation of such 3Gwireless technologies has not so far stretched much beyond busy city centers. In fact, GSM isstill the major technology for providing full coverage, while 3G technology is applied to coverhot-spot areas, mainly those with very high user densities. Thanks to multi-mode terminals,which can handle both standards (GSM and UMTS), wireless network users usually do noteven realize which technology they are currently using while making a call or using otherwireless services. Regarding the relevance of GSM technology, it is important to note thatmost network providers who have implemented UMTS are using basically the same fixedbackbone infrastructure architecture as used for GSM and GPRS together.

INTRODUCTION 3

1.2 The success of GSM

GSM is now in more countries than McDonalds.

(Mike Short, Chairman MoU Association 1995–1996)

The relevance of the GSM standard today becomes obvious when we take a brief look atthe success story of GSM so far and keeping in mind that many countries are still workingtowards full wireless coverage, mainly by deploying GSM. GSM was initially designed asa pan-European mobile communication network, but shortly after the successful start of thefirst commercial networks in Europe, GSM systems were also deployed on other continents(e.g. in Australia, Hong Kong and New Zealand). In the meantime, as of May 2008, 670networks in 208 countries are in operation according to GSM world.

In addition to GSM networks that operate in the 900 MHz frequency band, other so-called Personal Communication Networks (PCNs) and Personal Communication Systems(PCSs) are in operation. They use frequencies around 1800 MHz, or around 1900 MHz inNorth America. Apart from the peculiarities that result from the different frequency range,PCNs/PCSs are full GSM networks without any restrictions, in particular with respect toservices and signaling protocols. International roaming among these networks is possiblebased on the standardized interface between mobile equipment and the Subscriber IdentityModule (SIM) card, which enables personalization of equipment operating in differentfrequency ranges (SIM card roaming). Now that UMTS technology has been integrated bymost wireless providers into their networks, roaming not only between providers but alsobetween different technologies is already state of the art. To this end, multi-band and multi-standard terminals have been developed and are considered commonplace today. Users ofstate-of-the-art terminals with a SIM card from one of the major providers in Europe can usetheir terminals in different frequency ranges as well as in GSM and UMTS networks, withouthaving to configure or select anything. The terminals roam between different networks andtechnologies automatically.

1.3 Classification of mobile communication systems

This book deals almost exclusively with GSM; however, GSM is only one of many facets ofmodern mobile communication.

For the bidirectional – and hence genuine – communication systems, the simplest variant isthe cordless telephone with very limited mobility (particularly the Digital Enhanced CordlessTelecommunications (DECT) standard in Europe). This technology is also employed for theexpansion of digital Private Branch Exchanges (PBXs) with mobile extensions.

Local Area Networks (LANs) have also been augmented with mobility functions: WirelessLANs (WLANs) have been standardized and are now offered by several companies. WLANsoffer Internet Protocol (IP)-based, wireless data communication with very high bit ratesbut limited mobility. WLANs have been installed, for example, in office environmentsand airports, as a supplement or alternative to wired LANs, but also in universities, cafes,restaurants, etc. WLAN access points, however, are also very popular in private homes asaccess technology. In fact, in urban areas the coverage of IEEE 802.11 type access pointsis impressive and could theoretically be used for roaming while using WLAN by applying


Mobile IP enhanced routing for mobility support. This, however, is hindered by the fact thateach WLAN cell is typically managed by someone else, in effect making it impossible toform a large network. Another aspect is that most WLAN cells are of course encrypted andcannot therefore be used by just anyone. A little different are campus-type WLAN networks,operated by companies or universities, for instance. The IEEE 802.11 type WLAN standardsare continuously being amended. The IEEE 802.11n standard for high data rates enables datarates in the 100 Mbit/s range by applying multiple antennas and using multiple in multipleout (MIMO) technology. Even though standardization is not complete for IEEE 802.11n,so-called draft-n devices are already commercially available and promise data rates close to100 Mbit/s.

Another emerging class of wireless networks are being used for short-range commu-nication. Bluetooth, for example, replaces cables by enabling direct wireless informationexchange between electronic devices (e.g. between cellular phones, Personal Digital Assis-tants (PDAs), computers and peripherals). These networks are also called Body AreaNetworks or Personal Area Networks. Unlike the mobile technologies mentioned above, theyare not based on a fixed network infrastructure (e.g. base stations). The possibility of buildingup such networks in a spontaneous and fast way gave them the name ad hoc networks. WLANtechnologies also include the capability for peer-to-peer ad hoc communication (in additionto the classical client-to-base station transmission modus).

GSM and UMTS belong to the class of cellular networks that are used predominantly forpublic mass communication. These had an early success with analog systems such as theAdvanced Mobile Phone System (AMPS) in America, the Nordic Mobile Telephone (NMT)in Scandinavia, or the C-Netz in Germany. Founded on the digital system GSM (with itsvariants for 900, 1800 and 1900 MHz), a market with millions of subscribers worldwidewas generated, and it represents an important economic force. A strongly contributing factorto this rapid development of markets and technologies has been the deregulation of thetelecommunication markets, which allowed the establishment of new network operators.

Another competing or supplementary technology is satellite communication based on LowEarth Orbiting (LEO) or Medium Earth Orbiting (MEO) satellites, which also offer global,and in the long term even broadband, communication services. Trunked radio systems – indigital form with the European standard Trans European Trunked Radio (TETRA) – areused for business applications such as fleet control. They offer private services that are onlyaccessible by closed user groups.

In addition to bidirectional communication systems, there also exists a variety of unidi-rectional systems, where subscribers can only receive but not send data. With unidirectionalmessage systems (paging systems) users may receive short text messages. A couple of yearsago, paging systems were very popular, since they offered a cost-effective reachability withwide-area coverage. Today, the SMS in GSM has basically replaced the function of pagingsystems. Some billion SMS messages are being exchanged between mobile GSM userseach month. Digital broadcast systems, such as Digital Audio Broadcast (DAB) and DigitalVideo Broadcast (DVB), are very interesting for wireless transmission of radio and televisionstations as well as for audio- and video-on-demand and broadband transmission of Internetpages.

GSM and its enhancements (including UMTS air interfaces), however, will remain thetechnological base for mobile communication for many years, and will continue to open upnew application areas.

INTRODUCTION 5

1.4 Some history and statistics of GSM

In 1982 the development of a pan-European standard for digital cellular mobile radio wasstarted by the Groupe Spécial Mobile of the CEPT (Conférence Européenne des Adminis-trations des Postes et des Télécommunications) (see Table 1.1). Initially, the acronym GSMwas derived from the name of this group. After the founding of the European standardizationinstitute ETSI (European Telecommunication Standards Institute), the GSM group becamea technical committee of ETSI in 1989. After the rapid worldwide proliferation of GSMnetworks, the name has been reinterpreted as Global System for Mobile Communication.

After a series of incompatible analog networks had been introduced in parallel in Europe,e.g. Total Access Communication System (TACS) in the UK, NMT in Scandinavia and the C-Netz in Germany, work on the definition of a European-wide standard for digital mobile radiowas started in the early 1980s. The GSM was founded, which developed a set of technicalrecommendations and presented them to ETSI for approval. These proposals were producedby the Special Mobile Group (SMG) in working groups called Sub Technical Committees(STCs), with the following division of tasks: service aspects (SMG 01), radio aspects(SMG 02), network aspects (SMG 03), data services (SMG 04) and network operation andmaintenance (SMG 06). Further working groups were mobile station testing (SMG 07),integrated circuit card aspects (SMG 09), security (SMG 10), speech aspects (SMG 11) andsystem architecture (SMG 12) (ETSI, 2008). SMG 05 dealt with future networks and wasresponsible for the initial standardization phase of the next generation of the European mobileradio system, the UMTS. Later, SMG 05 was closed, and UMTS became an independentproject and technical body of ETSI. The Third Generation Partnership Project (3GPP) hasbeen founded in cooperation with other standardization committees worldwide. Its goalwas the composition of the technical specifications for UMTS. Finally, in July 2000, ETSIannounced the closure of the SMG which has been responsible for setting GSM standards forthe last 18 years. Their remaining and further work has been transferred to groups inside andoutside ETSI; most of the ongoing work has been handed over to the 3GPP.

After the official start of the GSM networks during the summer of 1992, the number ofsubscribers increased rapidly such that during the fall of 1993 already more than one millionsubscribers had made calls in GSM networks, more than 80% of them in Germany. On aglobal scale, the GSM standard also received very fast recognition, as evident from the factthat at the end of 1993 several commercial GSM networks started operating outside Europe,in Australia, Hong Kong and New Zealand. Afterwards, GSM was introduced in Brunei,Cameroon, Iran, South Africa, Syria, Thailand, USA and United Arab Emirates. Whereasthe majority of the GSM networks operate in the 900 MHz band (GSM900), there are alsonetworks operating in the 1800 MHz band (GSM1800) – PCN and Digital CommunicationSystem (DCS1800) – and in the United States in the 1900 MHz band (GSM1900) – PCS.These networks use almost completely identical technology and architecture; they differessentially only in the radio frequencies used and the pertinent high-frequency technology,such that synergy effects can be taken advantage of, and the mobile exchanges can beconstructed with standard components.

In parallel to the standardization efforts of ETSI, in 1987 the then existing prospectiveGSM network operators and the national administrations formed a group whose memberssigned a common Memorandum of Understanding (MoU). The MoU Association wassupposed to form a base for allowing the transnational operation of mobile stations using


Table 1.1 Time history – milestones in the evolution of GSM.

Year Event

1982 Groupe Spécial Mobile established by the CEPT.1986 Reservation of the 900 MHz spectrum band for GSM agreed in the

EC Telecommunications Council.Trials of different digital radio transmission schemes and differentspeech codes in several countries.

1987 Basic parameters of the GSM standard agreed in February.1988 Completion of first set of detailed GSM specifications for infrastructure.1989 Groupe Spéciale Mobile (transferred to an ETSI technical committee)

defines the GSM standard as the internationally accepteddigital cellular telephony standard.

1990 GSM adaptation work started for the DCS1800 band.1991 First GSM call made by Radiolinja in Finland.1992 First international roaming agreement signed between

Telecom Finland and Vodafone (UK).First SMS sent.

1993 Telstra Australia becomes the first non-European operator.Worlds first DCS1800 (later GSM1800) network opened in the UK.

1994 GSM Phase 2 data/fax bearer services launched.GSM MoU membership surpasses 100 operators.GSM subscribers hit one million.

1995 117 GSM networks on air.The number of GSM subscribers worldwide exceeds 10 million.Fax, data and SMS services started, video over GSM demonstrated.The first North American PCS 1900 (now GSM 1900) network opened.

1996 First GSM networks in Russia and China go live.Number of GSM subscribers hits 50 million.

1997 First tri-band handsets launched.1998 Number of GSM subscribers worldwide over 100 million.1999 WAP trials begin in France and Italy.2000 First commercial GPRS services launched.

First GPRS handsets enter the market.Five billion SMS messages sent in one month.

2001 First 3GSM (W-CDMA) network goes live.Number of GSM subscribers exceed 500 million worldwide.

2003 First EDGE networks go live.Membership of GSM Association breaks through 200-country barrier.Over half a billion handsets produced in a year.

2008 GSM surpasses three billion customer threshold.

INTRODUCTION 7

internationally standardized interfaces. As of April 2008, the GSM MoU has 747 memberswhich operates 670 GSM networks in 200 countries.

1.5 Overview of the book

The remainder of this book is organized as follows. In Chapter 2, we give an introductionto radio channel characteristics and the cellular principle. The understanding of duplex andmultiple access schemes serves as the basis for understanding GSM technology. We alsodescribe some measures to increase the capacity in GSM systems, sectorization, as appliedby most GSM networks already today, and Spacial Filtering for Interference Reduction(SFIR). Chapter 3 introduces the GSM system architecture and addressing. It explains thebasic structure and elements of a GSM system and their interfaces as well as the identifiersof users, equipment and system areas. Next, Chapter 4 deals with the physical layer atthe air interface (how are speech and data transmitted over the radio channel?). Amongother things, it describes GSM modulation, multiple access, duplexing, frequency hopping,the logical channels and synchronization. Also we discuss GSM coding (source coding,speech processing and channel coding). In Chapter 5, the entire protocol architecture ofGSM (payload transport and signaling) is covered. For example, communication protocolsfor radio resource management, mobility management, connection management at the airinterface are explained as well as mechanisms for authentication and encryption. Chapter 6describes in detail three main principles that are needed for roaming and switching: locationregistration and update (i.e. how does the network keep track of the user and find them whenthere is an incoming call?), connection establishment and termination and handover (i.e.how is a call transferred between cells?). Chapter 8 is on enhanced data services in GSM.It explains in detail GPRS which can be used for wireless Internet access. In addition thischapter includes HSCSD and Enhanced Date Rates for Global Evolution (EDGE). Chapter 7contains the major GSM services and, finally, Chapter 9 gives a brief outlook on futuremobile network developments. Appendix A covers basic GSM data services and Appendix Bdescribes network operation and management.

2

The mobile radio channel and thecellular principle

Many measures, functions and protocols in digital mobile radio networks are based onthe properties of the radio channel and its specific qualities, in contrast to informationtransmission through guided media. For the understanding of digital mobile radio networks itis therefore helpful to know a few related basic principles. For this reason, the most importantfundamentals of the radio channel and of cellular and transmission technology are presentedand briefly explained in the following. For a more detailed treatment, see, for example,Bertsekas and Gallager (1987), Lee (1989), Proakis (1995) and Steele and Hanzo (1999).

2.1 Characteristics of the mobile radio channel

The electromagnetic wave of the radio signal propagates under ideal conditions in free spacein a radial-symmetric pattern. The received power Pr decreases with the square of the distanceL from the transmitter. Specifically, the received power Pr can be described according to thefree-space model as a function of the transmit power Pt, the distance L and the wavelengthof the radio signal λ as

Pr = Pt · gt · gr ·(

λ

4πL

)2, (2.1)

where gt and gr are the transmit and receive antenna gains, respectively. While this model isappropriate, for instance, for inter-satellite as well as for Earth-to-satellite communication,it does not capture the effects of terrestrial radio propagation, where the signal is scatteredand reflected by obstacles such as buildings, mountains, vegetation, the ground and watersurfaces. At the receiver, direct and – potentially many – reflected signal components aresuperimposed. In effect, we can describe Pr as a linear function of Pt, gt, gr, and an overallchannel gain gc:

Pr = gc · gt · gr · Pt. (2.2)GSM – Architecture, Protocols and Services Third Edition J. Eberspächer, H.-J. Vögel, C. Bettstetter and C. Hartmannc© 2009 John Wiley & Sons, Ltd


The channel gain gc can be split into three components

gc = gd(L) · gs · gm (2.3)each capturing one of the main propagation effects.

• Distance-dependent path gain gd(L): This part of the channel gain is usuallymodeled as a deterministic function of the distance L between the transmitter andthe receiver, such that gd(L) · Pt gives the mean received power at distance L from thetransmitter (assuming gt = gr = 1). A common model for the path gain is given by

gd(L) =(

λ

4πL

)2(L0

L

)γ−2∼ L−γ , (2.4)

where L0 is a reference distance and γ ≥ 2 is the attenuation exponent, depending onthe propagation environment (Rappaport, 2002). Typical values for γ are between 3and 5. In addition to the described model, specifically for modeling and planning ofGSM networks, measurement-based models are available, such as the Okumura–Hatamodel (Hata, 1980; Okumura, 1968) for GSM900 networks and the COST-231 Hatamodel (Damosso, 1999) for GSM1800 networks. Those models are parameterized bythe heights of transmit and receive antennas as well as by the propagation environment(rural, sub-urban or urban).

• Shadowing gain gs: Shadowing describes the effect of fluctuations of the receivedpower around the main value, as it is caused by obstacles such as buildings andvegetation. The severeness of the shadowing effect depends on the number andproperties of obstacles between the transmitter and receiver. Changes in shadowingoccur in the order of meters, e.g. when a user turns around a corner during a phone call.In accordance with measurement data, the most commonly used model for shadowingis a statistical model, describing the shadowing gain gs as a log-normal distributedrandom variable. Therefore, the shadowing gain in decibels, i.e. χ = 10 log10(gs), isdistributed according to a Gaussian distribution given by

fχ (χ) = 1√2πσ

· e−χ2/2σ 2 . (2.5)

The standard deviation σ defines the severeness of the shadowing and depends onthe environment to be modeled. According to measurements, typical values for σ arebetween 5 and 10 dB (Geng and Wiesbeck, 1998).

• Multipath fading gain gm: Another source of received power fluctuations around themean value is caused by multipath propagation. In urban environments, in particular,multiple copies of the transmitted signal arrive at the receiver through differentpropagation paths. The superposition of many such copies of the transmitted signal,arriving at the receiver from different directions and with different delays, causesa wave field around the receiver. The received signal strength within this wavefield changes severely in the order of the signal wavelength between places wheredestructive and constructive superposition occurs. The resulting amplitude variations

THE MOBILE RADIO CHANNEL AND THE CELLULAR PRINCIPLE 11

Figure 2.1 Typical signal in a channel with Rayleigh fading.

are modeled by a random variable a, such that

gm = a2. (2.6)The distribution of the random variable a depends on the propagation environment.If no direct line of sight between sender and receiver is present, a is assumed to beRayleigh distributed, while an additional line of sight can be taken into consideration ifa Rice distribution is applied. Figure 2.1 shows typical channel fluctuations accordingto Rayleigh fading for a receiver traveling through the wave field. It can be shown thatif a is Rayleigh distributed, the multipath fading gain gm = a2 will be exponentiallydistributed (Schwartz, 2005).

The signal level observed at a specific location is determined by the phase shift of themultipath signal components. This phase shift depends on the wavelength of the signal, andthus the signal level at a fixed location is also dependent on the transmission frequency.Therefore, the fading phenomena in radio communication are also frequency specific. Ifthe bandwidth of the mobile radio channel is small (narrowband signal), then the wholefrequency band of this channel is subject to the same propagation conditions, and the mobileradio channel is considered frequency-nonselective. On the other hand, if the bandwidth of achannel is large (broadband signal), the individual frequencies suffer from different degreesof fading (Figure 2.2) in which case we speak of a frequency-selective channel (David andBenkner, 1996; Steele, 1992). Signal breaks because of frequency-selective fading along asignal path are much less frequent for a broadband signal than for a narrowband signal,because the fading holes only shift within the band and the received total signal energyremains relatively constant (Bossert, 1991).

In addition to frequency-selective fading, the different propagation times of the individualmultipath components also cause time dispersion on their propagation paths. Therefore,signal distortions can occur due to interference of one symbol with its neighboring symbols(‘intersymbol interference’). These distortions depend first on the spread experienced by a


Figure 2.2 Frequency selectivity of a mobile radio channel.

pulse on the mobile channel, and second on the duration of the symbol or of the intervalbetween symbols. Typical multipath channel delays range from 0.5 µs in urban areas to about16 to 20 µs in hilly terrain, i.e. a transmitted pulse generates several echoes which reach thereceiver with delays of up to 20 µs. In digital mobile radio systems with typical symboldurations of a few microseconds, this can lead to smearing of individual pulses over severalsymbol durations.

Owing to the described effects of the wireless channel, mobile information transportrequires additional, often very extensive measures, which compensate for the effects ofmultipath propagation. First, an equalizer is required, which attempts to eliminate thesignal distortions caused by intersymbol interference. The operational principle of suchan equalizer for mobile radio is based on the estimation of the channel pulse response toperiodically transmitted, well-known bit patterns, known as the training sequences (Bertsekasand Gallager, 1987; Watson, 1993). This allows the time dispersion of the channel and itscompensation to be determined. The performance of the equalizer has a significant effect onthe quality of the digital transmission. On the other hand, for efficient transmission in digitalmobile radio, channel coding measures are indispensable, such as forward error correctionwith error-correcting codes, which allows the effective bit error ratio to be reduced to atolerable value (about 10−5 to 10−6). Further important measures are transmitter powercontrol and algorithms for the compensation of signal interruptions in fading, which maybe of such a short duration that a disconnection of the call would not be appropriate.

2.2 Separation of directions and duplex transmission

The most frequent form of communication is the bidirectional communication which allowssimultaneous transmitting and receiving. A system capable of doing this is called full-duplex. One can also achieve full-duplex capability if sending and receiving do not occursimultaneously but switching between both phases is done so fast that it is not noticed


by the user, i.e. both directions can be used quasi-simultaneously. Modern digital mobileradio systems are always full-duplex capable. Essentially, two basic duplex proceduresare employed: Frequency Division Duplex (FDD) using different frequency bands in eachdirection, and Time Division Duplex (TDD) which periodically switches the direction oftransmission.

2.2.1 Frequency Division DuplexThe frequency duplex procedure has been used already in analog mobile radio systemsand is also used in digital systems. For communication between a mobile and a basestation, the available frequency band is split into two partial bands, to enable simultaneoussending and receiving. One partial band is assigned for uplink (from mobile to base station)transmissions and the other partial band is assigned for downlink (from base station to mobile)transmissions.

• Uplink band: transmission band of the mobile and receiving band of the base station.• Downlink band: receiving band of the mobile and transmission band of the base

station.

To achieve good separation of both directions, the partial bands must be a sufficient frequencydistance apart, i.e. the frequency pairs of a connection assigned to uplink and downlink musthave this distance band between them. Usually, the same antenna is used for sending andreceiving. A duplexing unit is then used for the directional separation, consisting essentiallyof two narrowband filters with steep flanks (Figure 2.3). These filters, however, cannot beintegrated, so pure frequency duplexing is not appropriate for systems with small compactequipment (David and Benkner, 1996).

2.2.2 Time Division DuplexTime duplexing is therefore a good alternative, especially in digital systems with timedivision multiple access. In this case, the transmitter and receiver operate only quasi-simultaneously at different points in time, i.e. the directional separation is achieved byswitching in time between transmission and reception, and thus no duplexing unit is required.Switching occurs frequently enough that the communication appears to be over a quasi-simultaneous full-duplex connection. However, out of the periodic interval T available forthe transmission of a time slot only a small part can be used, so that a time duplex systemrequires more than twice the bit rate of a frequency duplex system.

2.3 Multiple accessThe radio channel is a communication medium shared by many subscribers in one cell.Mobile stations compete with one another for the frequency resource to transmit their infor-mation streams. Without any other measures to control simultaneous access of several users,collisions can occur (multiple access problem). Since collisions are very undesirable for aconnection-oriented communication like mobile telephony, the individual subscribers/mobilestations must be assigned dedicated channels on demand. In order to divide the availablephysical resources of a mobile system, i.e. the frequency bands, into voice channels, specialmultiple access procedures are used which are presented in the following (Figure 2.4).


Figure 2.3 Frequency and time duplex.

Figure 2.4 Multiple access procedures.

2.3.1 Frequency Division Multiple Access

Frequency Division Multiple Access (FDMA) is one of the most common multiple accessprocedures. The frequency band is divided into channels of certain bandwidth such that eachconversation is carried on a different frequency (Figure 2.5). The effort in the base station torealize an FDMA system is very high. Even though the required hardware components are


Figure 2.5 Channels of an FDMA system.

relatively simple, each channel needs its own transceiving unit. Furthermore, the tolerancerequirements for the high-frequency networks and the linearity of the amplifiers in thetransmitter stages of the base station are quite high, since a large number of channels needto be amplified and transmitted together (David and Benkner, 1996; Steele, 1992). One alsoneeds a duplexing unit with filters for the transmitter and receiver units to enable full-duplexoperation, which makes it hard to build small, compact mobile stations, since the requirednarrowband filters can hardly be realized with integrated circuits.

2.3.2 Time Division Multiple Access

Time Division Multiple Access (TDMA) is used in digital mobile radio systems. Theindividual mobile stations are cyclically assigned a frequency for exclusive use only for theduration of a time slot, which obviously requires frame synchronization between transmitterand receiver. Furthermore, in most cases the whole system bandwidth for a time slot is notassigned to one station, but the system frequency range is subdivided into subbands, andTDMA is used for multiple access to each subband. The subbands are known as carrierfrequencies, and the mobile systems using this technique are designated as multicarriersystems (not to be confused with multicarrier modulation). GSM employs such a combinationof FDMA and TDMA; it is a multicarrier TDMA system. The available frequency range isdivided into frequency channels of 200 kHz bandwidth each (with guard bands between toease filtering), with each of these frequency channels containing eight TDMA conversationchannels.

Thus, the sequence of time slots assigned to a mobile station represents the physicalchannels of a TDMA system. In each time slot, the mobile station transmits a data burst.The period assigned to a time slot for a mobile station thus also determines the number ofTDMA channels on a carrier frequency. The time slots of one period are combined into aso-called TDMA frame. Figure 2.6 shows five channels in a TDMA system with a period offour time slots and three carrier frequencies.

The TDMA signal transmitted on a carrier frequency in general requires more bandwidththan an FDMA signal; this is because with multiple time use, the gross data rate has to be


Figure 2.6 TDMA channels on multiple carrier frequencies.

correspondingly higher. For example, GSM systems employ a gross data rate (modulationdata rate) of 271 kbit/s on a subband of 200 kHz, which amounts to 33.9 kbit/s for each ofthe eight time slots.

Narrowband systems are particularly susceptible to frequency-selective fading (Figures 2.1and 2.2) as already mentioned, such that a single channel might be in a deep fade whileswitching to another channel might result in a significantly better reception. Furthermore,there are also frequency-selective co-channel interferences, which can contribute to thedeterioration of the transmission quality. To this end a TDMA system offers very goodopportunities to attack and drastically reduce such frequency-selective interference byintroducing a frequency hopping technique. With this technique, each burst of a TDMAchannel is transmitted on a different frequency (Figure 2.7).

In this technique, selective interference on one frequency at worst hits only every ithtime slot, if there are i frequencies available for hopping. Thus, the signal transmitted bya frequency hopping technique uses frequency diversity. Of course, the hopping sequencesmust be orthogonal, i.e. one must ascertain that two stations transmitting in the same timeslot do not use the same frequency. Since the duration of a hopping period is long comparedwith the duration of a symbol, this technique is called slow frequency hopping. With fastfrequency hopping, the hopping period is shorter than a time slot and is of the order of asingle symbol duration or even less. This technique belongs to the family of spread spectrumtechniques. As mentioned above, for TDMA, synchronization between a mobile and basestation is necessary. This synchronization becomes even more complex due to the mobilityof the subscribers, because they can stay at varying distances from the base station and theirsignals thus incur varying propagation times. First, the basic problem is determining the exactmoment when to transmit. This is typically achieved by using one of the signals as a timereference, such as the signal from the base station (downlink, Figure 2.8). On receiving theTDMA frame from the base station, the mobile can synchronize and transmit a time slot

Documents

GSM – Architecture, Protocols and ServicesGSM – Architecture, Protocols and Services Third Edition Jörg Eberspächer Technische Universität München, Germany Hans-Jörg Vögel